1. Field of the Invention
The present invention relates to collision avoidance radar, and more particularly to a radar for use in automotive radar.
2. Description of the Prior Art
Conventional non-cooperative radar detecting and ranging systems, wherein a probe signal is transmitted toward a target and is "skin reflected" therefrom back to the interrogating unit, have typically been impractical for utilization as automotive collision avoidance systems because of what is known in the art as "clutter." "Clutter returns" from extraneous targets such as fixed objects along a roadway or moving vehicles in noncritical spacial relationship to the interrogating vehicle ("don't-care" targets) are not distinguishable from reflections from targets in critical relationship to the interrogating vehicle. The clutter returns thus cause a "false alarm" problem whereby automatic braking or passenger restraints such as air bags may be inopportunely deployed.
Cooperative systems wherein the radar is responsive only to specifically tagged targets which are capable of generating a reply signal, that in some way is distinguishable or discriminated from the radar interrogation and from skin reflections, are not subject to clutter. Such a cooperative detecting and ranging system, is disclosed in U.S. Pat. No. Re. 28,302 to Staras et al., dated Jan. 14, 1975. Such systems, however, do not respond to untagged targets or to targets having inoperative tags. Hence, the efficacy of such cooperative systems, as automotive detecting and ranging systems, is dependent upon acceptance and use of the system by the driving public at large and upon proper maintenance of the tags.
A vehicular collision avoidance system wherein forward viewing vehicle-mounted radio distance sensors are responsive to skin reflections from proximate targets and are adapted to cooperate with similar vehicle-mounted rearward viewing sensors that provide, in response to signals from the forward viewing sensor, actively augmented reflected signals to the forward viewing sensor, to thus allow detection of cooperating vehicles at greater distances while still using low power transmissions, is described in U.S. Pat. No. 3,760,414, issued Sept. 18, 1973 to A. M. Nicolson. The Nicolson system, thus, requires two transceivers, one in the forward facing sensor, and one in the rearward facing sensor. The sensors transmit regular trains of extremely short duration, relatively low amplitude baseband pulses. Such a system is disadvantageous in that there is no discrimination between active and passive targets and thus, if the Nicolson system is not restricted to short range operation, clutter due to returns from non-critical or "don't-care" targets is not provided for, thus causing false alarms and inopportune deployment of passenger restraints.
The present invention overcomes disadvantages and problems present in the prior art by providing a detecting and ranging system which responds both to tagged targets and to skin reflections for proximate untagged targets, and which is relatively immune to clutter.